In the mid-1980s, a study compared mtDNA from people around the world. It found that people of African descent (后裔) had twice as many genetic differences from each other than as did others. Because mutations (基因突变) seem to occur at a steady rate over time, scientists were able to conclude that modern humans must have lived in Africa at least twice as long as anywhere else. They now calculate that all living humans descend from a single woman who lived roughly 150,000 years ago in Africa, “Eve”. If geneticists are right, all of humanity is linked to Eve through an unbroken chain of mothers. This Eve was soon joined by “Y-Chromosome (染色体) Adam”, the genetic father of us all, also from Africa. DNA studies have confirmed that all the people on Earth, with all their shapes and colors, can trace their ancestry to ancient Africans.
What seems certain is that at a remarkably recent date—probably between 50,000 and 70,000 years ago—one small group of people, the ancestors of modern humans outside of Africa, left Africa for western Asia, either by migrating around the northern end of the Red Sea or across its narrow southern opening.
Once in Asia, genetic evidence suggests, the population split. One group stopped temporarily in the Middle East, while the other commenced a journey which would last tens of thousands of years. Moving a little further with each new generation, they followed the coast around the Arabian Peninsula, India, and Southeast Asia, all the way to Australia. ''The movement was probably unnoticeable,'' says Spencer Wells. ''It was less of a journey and probably more like walking a little farther down the beach to get away from the crowd.''
Although archaeological evidence of this 13,000-kilometer (8,000-mile) migration from Africa to Australia has almost completely disappeared, genetic traces of the group that made the trip do exist. They have been found in the DNA of native peoples in the Andaman Islands near Myanmar, in Malaysia, and in Papua New Guinea, and in the DNA of nearly all Australian aborigines (土著). Modern discoveries of 45,000-year-old bodies in Australia, buried at a site called Lake Mungo, provide some physical evidence for the theories as well.
People in the rest of Asia and Europe share different but equally ancient mtDNA and mutations. The mutations which they possess show that most are descendants of the group that stayed in the Middle East for thousands of years before moving on. Perhaps about 40,000 years ago, modern humans first advanced into Europe.
12. What can be inferred from the first paragraph?
A.Mutations occasionally take place over time |
B.Modern humans probably have only one ancestor. |
C.Modern humans must have lived in Africa for a shorter time than anywhere else. |
D.Scientists confirm all of humanity is linked to Eve with physical evidence . |
13. What happened to the first group of humans that moved from Africa into Asia?
A.Most of the migrants turned back into Africa. |
B.They separated into two groups. |
C.Most of the migrants moved directly into Europe. |
D.They stayed in the Middle East for tens of thousands of years. |
14. Which of the following is cited as evidence for the great migration to Australia?
A.Discovery of human remains in Australia | B.DNA of people in Southeast Africa |
C.DNA of immigrants to Australia | D.Discoveries from modern societies in Asia |
15. The title for this reading could be _______.
A.Finding Y-Chromosome Adam | B.Who were the First Humans? |
C.The Discovery of DNA in Africa | D.Migrating Out of Africa |